Telemeter



April 1947. J. L. ZAR 2,418,685 I I TELEMETER Filed April 19, 1944 2 Sheets-Sheet l uwmozoo am 200 320 360 DEGREES 6 /////A l- /III 24 FIELD STRENGH l6 0 3o 60 90 120450 \50 2w 240 270 500 330 560 .osmzess 1 Znnentor Jacob I... Zar

[LAM/W Gttomeg April 8, 1947.

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FIELD STENGFTH (H) 2 t 8 8 J. L. ZAR

TELEMETER Filed April 19, 1944 2 Sheets-Sheet 2 N \ZOIBO IMZ O 027030 350360 menses Q Zmmntcr Jacob L- 2a.:-

Patented Apr. 8, 1947 TELEMETER Jacob L. Zar, Orange, N. J., assignor to Thomas A. Edison, Incorporated, West Orange, N. corporation of New Jersey Application April 19, 1944, Serial No. 532,024

13 Claims. 1

This invention relates to telemetric systems and more particularly to a novel and improved telemetric system of the three-phase type which employs a transmitter of the single brush or single contact form and a receiver utilizing a permanent magnet rotor.

It is an object of my invention to provide novel telemetric systems of the character mentioned which have a greater accuracy in adhering to a linear response characteristic than has been heretofore obtained.

It is another object to provide novel telemetric systems of the character mentioned which have substantially linear response characteristics throughout a 360 scale range.

Another object is to provide novel and improvedreceiving instruments for telemetric systems of the character mentioned.

Other objects are to provide telemetric systems of the character mentioned which are efficient and reliable, and which are adapted to permit wide manufacturing tolerances in their several components.

In my invention, the rotor of the receiver is controlled wholly by the field coils without the use of any intervening magnetic materials between the rotor and the surrounding magnetic shield, it being of course understood that the field coils are mounted on suitable non-magnetic cores or supports. According to my invention I may provide innumerable designsof such receivers to meet different conditions, each of which will reproduce accurately and linearly the settings of the transmitter. By my invention, I am able to prescribe the physical parameters of these different designs. For instance I find that the desired torque characteristic of the receiver can be prescribed in terms of the electrical circuit parameters of the telemetric system and that the dimensions and locations of the-field coils can be prescribed in terms of the magnetic charac teristics of the rotor receiver.

These and other objects and features of my invention will more fully appear from the following description and the appended claims.

In the description of my invention reference is had to the accompanying drawings, of which:

Figure 1 is a circuit diagram of the type of telemetric system to which my invention relates;

Figure 2 is a graph showing one prescribed torque characteristic for the receiver of the telemetric system;

Figure 3 is a graph showing the percentage third harmonic in the torque characteristic rela- 5 2 tive to the electrical parameters of the circuit of the telemeter;

Figure 4 is a view illustrating one form of receiving instrument according to my invention;

Figure 5 is a plan view of a preferred shape of magnet for use with the receiving instrument of Figure 4;

Figure 6 is an elevational view of the magnet of Figure 5;

Figure '7 is a graph showing the approximate flux distribution for the magnet-shield arrangement of Figures 5 and 6;

Figure 8 is a diagrammatic view showing a second embodiment of receiver according to my invention;

Figure 9 is an elevational view showing a preferred shape of magnet .Ior the receiver of Figure 8;

Figure 10 is a graph showing the approximate flux distribution for the magnet-shield arrangement of Figure 9; v

Figure 11 is a view showing a third embodiment of receiver according to my invention wherein two coils are used per phase;

Figure 12 is a plan view of a preferred shape 'of magnet for use with the receiver of Figure 11;

Figure 13 is an elevational view of the magnet of Figure 12;

Figure 14 is a graph showing the approximate flux distribution for the magnet-shield arrangement of Figures 12 and 13; and

Figure 15 is a view of a receiver having the operating characteristics of the receiver of Figure 11 but with a different coil arrangement.

In Figure 1 there is shown a circuit diagram of the type of telemetric system above-mentioned. This telemetric system includes a transmitter T and a receiver R. The transmitter comprises a closed circular resistor which may be a wire helix II. This helix has uniformly distributed resistance and is provided with three taps 2 equidistantly spaced at intervals. At the center 3 of the helix H there is provided a single brush or contact arm 4 which slidably engages the helix. The receiver R comprises a rotor magnet 5 pivoted at B, and three similar field coils 1- or sets of coils as will hereinafter appear-which are spaced equidistantly about the magnet at 120 intervals. The coils are star connected (specifically, in the form of an electrical Y) to the respective taps 2 by lead wires 8 and the common junction 9 of the coils is connected by a lead wire In and through a battery B to the contact arm 4. The field coils are to have substantially equal resistances, the degree of variation which may be tolerated between coils depending upon the accuracy desired; and to compensate for variations between coils in manufacturing there may be inserted a small resistance la in series with any one of the field coils of lower resistance.

It will be understood, as is well known in electric circuit theory, that for each position of the contact 4 the magnetic fields 01' the three field coils combine to produce a resultant magnetic field, and that the direction of this resultant field will vary with the positioning of the contact arm. The rotor magnet, which is magnetized on an axis at right angles to its pivot axis, will assume the position wherein its magnetic axis is aligned with the direction of that res utant field. Accordingly, as the contact arm 4 is turned the rotor will turn likewise, and the corresponding positions of the rotor will be indicated by a pointer i2 relative to a scale II on a dial 44. Since in this telemetric system the currents in the field coils represent three phases, in respect of their spatial relationship about the pivot axis of the receiver as a center, the system is commonly referred to as being of the threephase type. For reasons which are apparent from the foregoing description, the transmitter is known as of a single-contact type and the receiver as of the permanent-magnet-rotor type.

In telemetric systems it is desired that the rebered harmonics.

ceiver pointer shall indicate accurately the positioning of the transmitter arm or contact throughout the range of travel 01' the latter. When the pointer of the receiver assumes positions corresponding substantially exactly with those of the transmitter arm throughout the whole scale range, then the scale divisions of the receiver may be evenly or linearly spaced, and the system will have a so-called linear response characteristic. It will of course be understood that a linear response is highly desirable, particularly in that the general positioning of the pointer can then be relied upon for an approximate reading without reference to any scale or dial; also, a linear characteristic facilitates manufacturing and calibration of the telemeter in production.

The diiferent designs of my invention are arrived at by varying the ratio of the resistances of the field coils to the resistance of the transmitter,-

by varying the shape of the rotor magnet and by varying the shape and location of the field coils. These parameters cannot however be varied independently of one another as they must all be held to a predetermined relation in order to obtain a linear response. Basically, the significance of the ratio between the resistances of the field coils and the transmitter is that it establishes the torque deflection characteristic of the receiver/ which is required for obtaining a linear response. As will be apparent from the following description, each such chosen or established torque deflection characteristic may be realized by innumerable-combinations of rotor magnets and field coils.

The term torque defiection characteristics" is used herein to mean the variation in restoring torque exerted by the field coils on the rotor magnet as the magnet is forcibly deflected from an equilibrium position. It will for example be understood that for each position of the transmitter arm 4 there is such position of equilibrium for the rotor magnet, this position being one wherein the magnetic axis of the magnet is aligned with the direction of the resultant magnetic field of the field coils. As the rotor magnet is forcibly deflected from such equilibrium position, a restoring torque is exerted by the interaction of the field coils on the magnet, to return it to that equilibrium position, which increases to a maximum and then falls oil to zero as the rotor magnet is deflected to from equilibrium position; at this point, the rotor magnet is in a condition 01' unstable equilibrium; With further deflection of the rotor magnet, the restoring torque increases to a maximum in the opposite direction and then again falls back to zero as the magnet is returned to its initial equilibrium position. This variation in restoring torque with defiection is thus ofa sinusoidal character which may be represented by a Fourier series. When the field coils, or sets of coils, are identical and the magnet is symmetrical to its pivot axiswhich are the conditions herein consideredthe shape. of the torque deflection characteristic, or torque curve as it may be termed, through the first 180 deflection from equilibrium position is the same, except for direction, as through the second 180 deflection, and the Fourier series thus comprises only a fundamental and odd-num- This Fourier series may be written as follows:

Torque sin 0+b sin 30 (1) where b is the fraction or percentage of third harmonic and a is the angle of deflection or the rotor magnet from equilibrium position. While this torque curve may include higher odd-numbered harmonics than the third, these higher harmonies have a lesser order of importance and need 'not be considered for purposes of my invention.

It is found that the shape of the torque curve determines the accuracy with which the receiver shall linearly follow the movement of the transmitter contact arm. This shape of the torque curve can be expressed in terms or the percentage of its third harmonic component. By my invention, I find that a substantially negligible error in linearity is obtained when where r is the resistance of each section of the helix ll between the taps 2 (3r being the resistance of the whole helix) and l is the resistance of each field coil 1. A graphical representation of this equation appears in Figure 3, where the ordinants represent the fraction oi third harmonic b and the abscissas represent the ratio of r/l. As a typical case each transmitter section may have a resistance equal to twice that of each field coil giving the ratio of r/l a value of 2. For this value of 1/1 there is required a coeificient of third harmonic b of approximately-5.2% oi the fundamental-as may be determined by reference to Equation 2 or the graph of Figure 3-in order to obtain a substantially linear response.

In Figure 2, there is shown a torque curve having approximately this percentage of third harmonic. This torque curve is to be obtained irrespective of the specificdesign of the receiver so long as the resistances of the transmitter sections are twice the resistances of the respective field coils. For other resistance ratios, different percentages of third harmonic are required according to the above Equation 2. I now show how the required torque curve is obtained by different specific designs of the receiver.

It will be realized that the only intrinsic difference between rotor magnets of difierent shapes material such as one of the Alnicos? considered under actual operating conditionsa that is, with a surrounding magnetic shield as is shown in the following specific embodiments. For any given rotor magnet and surrounding field, the fiux field distribution may be measured by interposing a conductor between the magnet and shield, rotating the magnet at uniform rate, and measuring the induced voltage in the conductor. A graph of the induced voltage will have the same shape as that of the fiux field. For symmetrical magnets, as are here considered, the magnetic field strength H may be represented by a Fourier series comprising a fundamental and odd-numbered harmonics Again, only the third harmonic need be considered, wherefore the magnetic field may be written as:

H=sin +d sin 3 5 (3) where d is the fraction of, third harmonic and is the angle between the magnet and the conductor abovementioned. This equation is wholly analogous to the torque curve of Equation 1.

Preferably, I use rotor magnets which are relatively long compared to the internal diameter of the surrounding shieldthat is, of the order of at least half the length of the shield diameteras then the tolerances on the magnet are less critical and a greater accuracy in response is obtained. The use of such flong magnets greatly facilitates the design of the receiver because with such magnets the fiux in the air gap between the magnet and the surrounding shield is confined closely to the plane of the magnet and the only part of the field coils which are instrumental in determining the response characteristic of the receiver are the longitudinal legs thereof which are intersected by the magnet fiux. Thus the only significant dimensions of the coils are those of their longitudinal legs; the transverse or crossover legs, being outside the magnetic field, may have any convenient shape without affecting the response of the receiver.

By my invention I am able to specify the positions and dimensions of the longitudinal legs of the field coils in terms of the fraction d of third harmonic in the flux field distribution and the fraction 2) of third harmonic in the torque curve. This relationship is now illustrated in terms or a specific embodiment of receiver according to my invention.

In Figures 4, 5 and 6 there is shown a receiver comprising a rotor magnet i 5 which is preferably made or a highly efiicient permanent magnetic The magnet ls pivoted for rotation about an axis l6. Sur rounding the magnet 4 is a shield or sleeve i! which is preferably made of a non-permanent and highly permeable magnetic material such as Mumetal." This is a cylindrical shield positioned concentrically to the pivot axis 16 so that the magnetic field of the rotor is the same for all positions thereof, and the rotor is without any magnetic bias. This insures that the same value of d (of Equation 3) holds for all positions of the rotor magnet. Within the shield ll there are three identical field coils l8. These coils are so positioned that the opposite legs l8a, hereinbefore referred to as the longitudinal legs, will lie par- (iii 6 allel to the pivot axis of the meter, between the magnet and shield, with the axes of the coils intersecting the pivot axis and spaced at intervals thereabout. The significant dimensions of the longitudinal legs 18a of each coil are their angular span and their angular widths. By "angular span" is meant the angle subtended by radial lines running from the pivot axis Hi to the centers of the legs l8a, and by the angular width is meant the-angle which the legs subtend about the pivot axis as a center. These significant dimensions of each field coil are given in my invention by the equation:

(3-4 sin 11:)(3-4 sin 9) d (4) where b and d are the fractions of third harmonic in the torque and fiux curves as aforementioned, g is one-half the angular width in degrees of the legs [8a of each coil and x is one-half the angular span in degrees of each coil as above defined. Or alternatively, by expressing b in terms of the ratio r/l, and transferring terms, this equation may be expressed as:

sin 2:)(3-4 sin g) (4a) These formulae give the relation between all the significant parameters of the telemeter system for obtaining a linear response.

It may be pointed out by way of example that Equation 4 may be used as follows: An approximate ratio of r/Z is first chosen-which ratio is not critical. This value is substituted in Equation 2 to find the value of third harmonic in the torque curve, which is the quantity b. An appropriate magnet is then chosen, and its fiux field distribution is measured as for example by the method aforedescribed. The flux curve is then analyzed, by standard Fourier analysis, to determine the fraction of third harmonic d. A value for the span of the field coils is then selected and, with these values, Equation 4 gives the angular width of the legs l8a which is required to obtain a linear response. I

A preferred span for the field coils is for with such coils the tolerances are less rigid and a maximum couplingcoefiicient is obtained to give a maximum torque efilciency. For this span of field coil there may be suitably used a magnet having an approximately ellipsoidal shape as shown in Figures 5 and 6. As a typical case, for a shield I! having a 1" inside diameter, the magnet may have a length along its major axis of a length along its minor axis of the ends of the magnet may terminate in arcs of 1% radii about the pivot axis as a center, and the sides may terminate on arcs of radii about the ends of the minor axis as centers; and the thickness of the magnet may be 1%" at the center and may terminate to 3 2" at theends. The magnet is charged along its major axis between two fiat poles with air gaps, and the major axis is then also the magnetic axis hereinbefore re ferred to.

The magnet above described has a flux distribution characteristic approximately as represented by the graph of Figure 7, which curve has a fraction of third harmonic d of the order of approxlmately --.053. On the basis that the torque curve of Figure 2 is to be obtained, the coefficient of third harmonic b is -.052 as aforementioned and for 180 coils the value of x is 90. Using these values in Equation 4 it is found that g shall be 7, which means that each coil leg I 8a shall be 14 wide. When the coil legs have this width, for the values above considered, there will be obtained a linear response.

It will of course be understood that as an alternative procedure the dimensions of the coils may be first assigned, in terms of their span and the width of their longitudinal legs, as also may be the value of r/Z, and that. then the value of d may be computed by the Equation 4a. Having determined d, I then find the shape of magnetwhich may be done by trial-that will have a flux curve with this fraction d of third harmonic.

Since the field coils have 180 span and are spaced 120 apart, their cross-over legs lab must neeessarly overlap. Preferably, I make the three coils identical but with one of the longitudinal legs 58a, of each coil longer than the other leg of the same coil so that, in the assembled meter, each coil will pass through one of the two remaining coils and will envelop the other remaining coil as indicated in Figure 4. Also, .1 preferably bow the cross-over legs lab outwardly from the pivot axis so as to provide a central opening 19 through which the rotor magnet may be passed without disturbing the coils.

In Figures 8 and 9 there is illustrated another receiver according to my invention wherein there is again obtained the torque deflection characteristic between the magnet and field coils .represented by Figure 2; hence, the value for b this design is again .052. This receiver comprises a magnet 20 pivoted at 2|, a surrounding concentric shield such as the shield IT and three identical field coils 22 which are interposed between the magnet and shield. The field coils are arcuately shaped so that they will lie wholly within the space between the magnet and the shield, but it will be understood that each coil may be made fiat and will have the same operating characteristics so long as the span of the coils and the width of their longitudinal legs remain the same. The rotor magnet l used with this embodiment is a fiat disk of Alnico" about thick having diametrically opposite end portions with fiat bevels 24. As typical values, for a shield of 1 diameter, the magnet may be ,1 in diameter, the thickness at the tip, s s", and the separation of the parallel lines 23 from where the bevels begin, This magnet is magnetized on a central line running through its beveled ends and has a fiux field distribution approximately as shown in Figure 10. By Fourier analysis, the fraction of third harmonic in this fiux distribution curve is +054.

On selecting a span for each coil of 90 (31 45") and using the values of .052 and +054 for b and d respectively, Equation 4 specifies that g shall be 10that is, that the width of the longitudinal legs 22a of each coil shall be 20. For these values, there will again be obtained a substantially linear response.

In Figures 11, 12 and 13 I show another embodiment of receiver according to my invention wherein there are empoyed two field coils per phase, or a total of six coils. This receiver comprises the cylindrical shield H, a centrally pivoted magnet 25 and three similar sets of coils 26, 21 and 28, of which each set comprises two coils connected preferably in series, the coils for set 26 being 26a and 2617, etc. In the circuit of Figure 1, these coil sets replace the single field coils per phase as used in the foregoing embodiments. The coil 26a passes through the coil 26b of its own set, and is accordingly made shorter in its dimension running axially of the meter, but all coils are alike in respect of their span about the pivot axis as a center and the width of their longitudinal legs 26c. Each coll set may be considered as having an effective magnetic axis which is analogous to the coil axis of each single coil considered in the foregoing embodiments. By way of illustration the effective axis for coil set 26 is designated as A in Figure 11. The angular spacing of these efiective axes is again at 120 intervals about the pivot axis. The coils of each set may be positioned at angles to the eflective axis A, but will be symmetrically disposed thereto as shown. The agnular span between the two longitudinal legs nearer the eflfective axis A may be termed 2.1:, and that between the other two legs which are farther from the axis A, 2w, 1: and w being then the angular spacing of the longitudinal legs from the eifectlve magnetic axis of the coil set. Using a, b and d with the same significance as before, I find by my invention that receivers using two coils per phase will have a substantially linear response when or, alternatively, by expressing b in terms 0! rl! and transposing terms, when The magnet 25 which is preferably used with this embodiment is shown in Figures 12 and 13. This magnet consists of a disk of Alnico" having diametrically opposite segment portions cut off to form a pair of peripheral fiats 25a. By way of example, the magnet may have a diameter of a length from flat to fiat of /z and a uniform thickness of The magnet is charged on a, diameter line running centrally through the fiats 25a.

The magnet above described has approximately the flux curve shown in Figure 14. This curve measures to have a fraction of third harmonic of +125. As typical and convenient values of the field coils, each coil of each set may have a span of 100 about the pivot axis and the two coils of each set may be spaced 60 apart-that is, at plus and minus 30 from the magnetic axis A of the set. Each coil set thus has a pair of legs nearer to the effective axis A which have a span 01 from each other and another pair farther from the axis A which have a span of 140 from each other. The value of x and w for each coil set is thus 30 and respectively. The angular width of each coil leg may be 16, giving a a value of 8- Using these values in Equation 5, with a value for d of as above mentioned, b computes to be .043. Using this value of b in Equation 2, the ratio r/l is computed to be approximately 1.5. This means the resistance of each transmitter section shall have approximately 1.5 times drical shield I! and magnet 25 as in the embodiment of Figure 11, and has three sets 30 of field coils of which each set consists of two coils 30a and 30b. In this case, however, the coils of each set are parallel and axially aligned with each other, the smaller coil 30a having a mean span 9 and the larger coil a span In the arrangement of of approximately 60 of approximately 140.

the three sets of coils in the assembled meter, a 1 leg of the larger coil of each set lies-between the the larger coils are made'oi diiierent length, ax-

'ially along the meter, their angular spans about the pivot axis being however the same. g The corresponding values of a: and to with this f coil arangement are accordingly 30 and 10, the

same as with the arrangement 01' Figure 11. Upon making the angular width of the coil legs also 16, will have a value of 8 and d will again be by Equat on equal to +l2.5%, g'ving there- .iore the same flux distribution characteristic as is shown in Figure 14. a The three specific embodiments hereinabove shown and described are examples of the various arangements possible by my invention, and are presented to illustrate the relationships between the s gn ficant parameters of the telemetric system which are basic to obtain ng a substantially linear response. It will be understood that innumerable other embodiments are possible within the range of these relationships. It will be further understood that these embodiments of my invention are n other respects illustrative and not limitative thereof and that within the range of engineering skill and by use of the equations herein presented various changes and modifications may bemade therein without departure from the scope of my invent on. which I endeavor to express according to the following claims.

1. In a telemetric system including three like transmitter resistances serially connected in closed circuit arrangement, and a single contact ior traversing said resistances and a D.-C. source or potential: a receiver comprising a pivoted perv manent magnet and a plurality of field coils spaced at intervals around the pivot axis oi-said magnet and connected as an electrical Y, the outer terminals of said Y being connected to the respective junctions between said transmitter re-- sistances, means connecting said source of potential between said contact and the center of field coils connected as an electrical Y and having three efiective magnetic axes spaced substantially equidistantly about the axisof rotation of said magnet, and a magnetic shield surrounding said field coil arrangement and substantially concentric to the pivot axis oi said magnet, the

space between said magnet and shield being free of magnetic materials, the outer terminals of said Y being connetced to the junctions between said transmitter resistances; and means connect- .ing said potential source between the center of said ,Y and said contact, said magnet and said coils being shaped to produce a restoring torque on said magnet which varies, upon said contact being held to a given position and the magnet being deflected from an equilibrium position, ac-

cording to a Fourier series'comprising a iundamental and a fraction or third harmonic equal substantially to where r/l is the ratio of resistance between the the magnet according to a Fourier series comprising a fundamental and a fraction of thir harmonic equal substantially to I .89r/l sing) (3-4 sin a:

where r/l is the ratio of the value of each trans mitterresistance to the resistance of each field coil, 1: is one hali the angular span in degrees of each field coil about said axis and g is one-- half the angular width in degrees 01 each leg of said coils about said axis.

said Y for causing said field coils tobe energized to produce a torque-directing influence on said magnet according to the positioning of said oontact, said magnet and said coils being shaped to produce a restoring torque on said magnet which varies, upon said contact being held at a giv n fixed position and the magnet being defiec d I from an equilibrium position, according to a Fourier series comprising a fundamental and a iraction or third harmonic equal substantially to where r/l is the ratio of the. value oi'each 0! said transmitter resistances to the resistance between 4. The telemetric system-set forth in claim 2 wherein said field coil arrangement consists of three similar coils, and said magnet is shaped to have a-flux field whose strength I! in the space between the magnet and said shield varies, at angles about said pivot axis from the magnetic axis of the magnet, substantially according to the equation sin +d sin 34:

(3 -4 sin" g)(s-4 3- where is one-half the angular width in degrees of each coil leg about said pivot axis, a: is onehalt the angular span in degrees of the legs of each coil about said pivot axis. and r/! is the the center and outer terminals respectively of tact for traversing said resistance element and a 11-0. source of potential: a receiver comprising a pivoted permanent magnet, an arrangement of ratio of the value or each of said transmitter resistances to the resistance of said respective field coils.

. 5'. In a telemetric system including a-trans-' mitter of the single-contact type: a receiver for said system comprising a pivoted permanent magnet, three similar field coils connected as an electrical Y and spaced angularly about the pivot axis 0! said magnet, and a cylindrical magnetic shield surrounding said coils and magnet,-eac h of said coils having substantialLv a span about said pivot axis as a center, said magnet being symmetrical to its pivot axis and having a set forth in claim 2- aua'eas substantially ellipsoidal shape as viewed along its pivot axis, and said magnet being charged along its major elliptical axis.

6. The telemetric system set forth in claim 2 wherein said field coil arrangement comprises three similar field coils each having a substantially 90 span about said pivot axis as a center, and wherein said magnet has a disk shape with beveled pole end portions.

7. The telemetric system set forth in claim 2 wherein said field coil arrangement consists of three like pairs held coils, and said magnet is shaped to have a flux iield whose strength varies around the magnet according to a Fourier series comprising a fundamental and a fraction ,0! third harmonic equal substantially to (3+r/l) (34 sin g)[3-4 (sin ::sin a sin w+sin' where r/l is the ratio of the value of each transmitter resistance to the resistance of each coll pair, a is one-half the angular width in degrees oi eachleg of each field coil which is in the space between the magnet and said shield, and: and w are the angular spacings in degrees between the effective axis 0! each coil pair and the coil legs of thatpair to each side of that axis.

8. The telemetric system set iorth in claim 2 wherein said coil arrangement comprises three pairs of field coils, each or said pairs having two legs at each side 01 the effective magnetic axis oi the pair of which one leg is spaced by approximately twice the angular distance from said axis than is the other leg, and wherein said magnet has the shape of a disk provided with two diametrically opposite peripheral flats and is charged on a diameter line running centrally through said flats.

9. The combination set forth in claim 1 wherein said receiver has three field coils spaced angularly at approximately 120 intervals and each coil has approximately a 180 span.

10. A teiemetric receiver for a telemetric system of the three-wire, single-brush transmitter type, comprising a permanent magnet pivoted on an axis through the center thereof with the magnetic axis of the magnet at substantially right angles to said pivot axis, three field coils positioned at 120 intervals about said pivot. axis with the magnetic axes of the coils radial to said pivot axis, said coils surrounding said magnet and spanning 180 respectively with respect to 12 said pivot axis. and amagnetic shield surrounding said coils and symmetrically disposed with respect to said pivot axis.

11. In a telemetric system 0! the three-wire type comprising a closed transmitter resistance having a single brush: the combination of a receiver comprising a pivoted permanent magnet having the magnetic axis thereof substantially at right angles to its pivot axis, three field coils surrounding said magnet respectively and spaced at intervals about said pivot axis, said coils having respectively spans about said pivot axis and being interconnected to have a neutral terminal, and a magnetic shield surrounding said coils and symmetrically disposed with respect to said pivot axis; and means for connecting a source of potential between said brush and said neutral terminal.

12 1a a telemetric system of the three-wire, single-brush transmitter type: a receiver comprising a pivoted permanent magnet having its magnetic axis crosswise to its pivot axis, three substantially identical field coils extending one through the other and spaced at 120 intervals about said pivot axis. said coils respectively having longitudinal legs spaced substantially 180' about said pivot axis with one longer than the other, and said coils having cross-over legs that overlap one another.

13. In a telemetric system of the three-wire,

nmmoas orran The following references are of record in the die of this patent:

, UNITED STATES PATENTS Number Name Date 1,281,658 Remy et al Oct. 15, 1918 2,330,588 Jewell Sept. 28, 1943 1,857,910 Allen et al Jan. 31, 1938 

